This invention relates to lustrous mica-flake pigments coated with TiO.sub.2 and SnO.sub.2.
A whole series of mica flake-based lustrous pigments coated with metal oxides has been known, e.g. from U.S. Pat. Nos. 3,087,828; 3,087,829; and 3,711,308; German patent specification No. 2,009,566 and German published specification No. 2,214,545. However, for special purposes, the light fastness and the weathering resistance of all of these pigments leaves something to be desired due primarily to the fact that in these pigments the TiO.sub.2 is present in the anatase modification which is evidently favored by the mica substrate.
There are also known mica flake-based lustrous pigments which contain both TiO.sub.2 and SnO.sub.2. The tin oxide is either present as a surface coating on a TiO.sub.2 coating or as a mixed precipitate with TiO.sub.2. See, e.g., German patent specification No. 1,467,468. X-ray investigation of such pigments shows that the TiO.sub.2, in spite of its tin content, is present in the anatase modification. According to published German specification No. 2,214,545, pigments are preferred in which the metal oxide layer consists preponderantly of rutile TiO.sub.2 plus a small amount of tin oxide, with the concentration of the tin oxide being greater in the regions closest to the mica. The SnO.sub.2 content of the metal oxide layer is preferably 0.5 to 5 wt. % but in no case to more than 20 wt. %. However, it has been found that these pigments contain TiO.sub.2 which is not completely and not reproducibly in the rutile form but instead is very frequently present at least partially in the anatase form, although it is stated in published German patent specification No. 2,214,545 that the TiO.sub.2 was present "substantially in rutile form."
Thus, it has been known for some time that, on the one hand, SnO.sub.2 acts as catalyst for the conversion of anatase into rutile and that, on the other hand, mica favors the formation of the anatase modification of the TiO.sub.2. Nevertheless, a satisfactory solution to the problem of producing rutile-containing pigments does not exist.
It is an object of this invention to provide lustrous pigments based on mica flakes coated with TiO.sub.2 having improved properties in which the TiO.sub.2 is present completely and reliably as rutile, even if the pigments have a comparatively thick layer of TiO.sub.2 exhibiting interference colors. It is another object of this invention to provide such pigments which, compared to comparable pigments in which the anatase modification is present, display considerable technical advantages in use, including better light and weathering fastness. It is still another object to provide processes for the production and use of such pigments. Other objects will become apparent to those skilled in the art.